This invention relates to the synthesis of aluminophosphate and silico-aluminophosphate molecular sieves, to aluminophosphate and silico-aluminophosphates having the CHA framework type and their use. In particular the present invention relates to the synthesis of aluminophosphate and silico-aluminophosphate molecular sieves using synthesis templates in combination with compounds, which have two or more fluorine substituents as a source of fluoride ions.
Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstocks. It has been known for some time that oxygenates, especially alcohols, e.g. methanol, are convertible into light olefin(s). The preferred methanol conversion process is generally referred to as methanol-to-olefin(s) (MTO) process, where methanol is converted to primarily ethylene and propylene in the presence of a molecular sieve.
Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloaluminophosphates such as the silicoaluminophosphates (SAPO""s). There are a wide variety of SAPO molecular sieves known in the art, of these the more important examples include SAPO-5, SAPO- 11, SAPO- 18, SAPO-34, SAPO-35, SAPO-41, and SAPO-56. For the methanol-to-olefins process SAPO molecular sieves having the CHA framework and especially SAPO-34 are particularly important catalysts. The CHA framework type has a double six-ring structure in an ABC stacking arrangement. The pore openings of the structure are defined by eight member rings that have a diameter of about 4.0 xc3x85, and cylindrical cages within the structure of approximately 10xc3x976.7 xc3x85 type (xe2x80x9cAtlas of Zeolite Framework Typesxe2x80x9d, 2001, 5th Edition, p. 96). Other SAPO molecular sieves of CHA framework type include SAPO-44, SAPO-47 and ZYT-6.
The synthesis of AlPO4 and SAPO molecular sieves is a complicated process. There are a number of variables, which need to be controlled in order to optimise the synthesis in terms of the purity, yield, and quality of the molecular sieve produced. Of these variables the choice of template (hereinafter also referred to as templating agent) is usually one of the most important in determining which framework type is obtained.
One desirable group of silicoaluminophosphate molecular sieves are those that have low silicon contents. Silicoaluminophosphates of the CHA framework type with low silicon contents are particularly desirable for use in the methanol-to-olefins process. Wilson, et al., reported that it is beneficial to have lower Si content for methanol-to-olefins reaction (Microporous and Mesoporous Materials, 29, 117-126, 1999). Low Si content has the effect of reducing propane formation and decreasing catalyst deactivation.
In U.S. Pat. No. 4,440,871 (Lok et.al) the synthesis of a wide variety of SAPO materials of various framework types are described with a number of specific examples. Also disclosed are a large number of possible organic templates, with some specific examples. In the specific examples a number of CHA framework type materials are described. The preparation of SAPO-34 is reported, using tetraethylammonium hydroxide (TEAOH), or isopropylamine, or mixtures of TEAOH and dipropylamine (DPA) as templates. Also disclosed in this patent is a specific example that utilises cyclohexylamine in the preparation of SAPO-44. Although other template materials are described in this patent there are no other templates indicated as being suitable for preparing SAPO""s of CHA framework type. Certain aminoalcohols are mentioned, including triethanolamine, N-methyldiethanolamine, N-methylethanolamine, N,N-dimethylethanolamine and N,N-diethylethanolamine as possible templates for SAPO molecular sieves. Of these materials N,N-diethylethanolamine is shown to produce SAPO-5, which is of framework type AFI. For the other aminoalcohols no indication is provided as to which SAPO or which framework type may be obtained through their use.
Since the synthesis of SAPO-34 was reported in U.S. Pat. No. 4,440,871, tetraethylammonium hydroxide (TEAOH) either alone, or in combination with dipropylamine (DPA), has been the preferred template for preparing SAPO-34. However, there are problems associated with the use of TEAOH and DPA. When used alone, TEAOH affords a limited range of synthesis parameters. For example, under certain conditions TEAOH will also template the synthesis of SAPO-18 which has the AEI framework type. TEAOH is thus relatively intolerant to synthesis condition variations. TEAOH is sometimes combined with DPA. However, DPA has a low boiling point (110xc2x0 C.) resulting in the need for production plants that can handle high pressures. In certain countries, the use of DPA requires special regulatory authorizations due to its toxicity. Also, DPA is an aggressive template and is often implicated in re-dissolution of the silicoaluminophosphate molecular sieve during its synthesis, resulting in poor quality crystalline product due to surface pitting of the crystals. Finally, it has proved difficult up to now to make pure phase CHA silicoaluminophosphate molecular sieves with a low silica to alumina ratio.
In U.S. Pat. No. 4,440,871, it was reported that SAPO-44 was obtained xe2x80x9cas the major phasexe2x80x9d using cyclohexylamine as a template. In U.S. Pat. No. 6,162,415 (Liu, et.al.), relatively pure CHA SAPO-44 was obtained using the same template but with control of the ratio of template to aluminium source and the ratio of phosphorous source to aluminium source.
In European Patent Publication No. 0,993,867, it was reported that the use of methylbutylamine resulted in SAPO-47 and the use of cyclohexylamine resulted in impure SAPO-44. Methylbutylamine has an even lower boiling point, at 91xc2x0 C., than DPA.
In U.S. Pat. No. 4,861,739 (Pellet, et al.), Example 102, it was reported that the use of N,N-diethylethanolamine produced CoAPSO-47, having Si concentrated on the peripheries of the crystal and Co at the centre.
In U.S. Pat. No. 4,310,440 (Wilson et.al), triethanolarnine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N-methyldiethanolamine, and N-methylethanolamine, were all used to prepare AlPO4-5, aluminophosphates of framework type AFI. N-methylethanolamine was also reported to produce AlPO4-21 of framework type AWO.
In European Patent Publication No. 0,993,867, it was reported that diethanolamine produced SAPO-34 and SAPO-5 under different synthesis conditions.
In the art various attempts have been made to improve the synthesis of AlPO4 or SAPO molecular sieves. One approach has been the addition of a source of fluoride ions to the synthesis mixture.
In U.S. Pat. No. 5,096,684 (Guth et.al.), morpholine and tetraethylammonium hydroxide were found to template the production of SAPO-34 when in the presence of HF. According to Guth et.al. the use of HF in combination with the organic template results in silicoaluminophosphates which have improved thermal and hydrolytic stability.
The use of morpholine in combination with hydrogen fluoride has also been described in: xe2x80x9cMultinuclear NMR Analysis of SAPO-34 Gels in the Presence and Absence of HF: The Initial Gelxe2x80x9d, O. B. Vistad, E. W. Hansen, D. E. Akporiaye, K. P. Lillerud: J. Phys. Chem A 103 (1999) 2540-2552.
In U.S. Pat. No. 4,786,487 (Kuehl et.al.), SAPO-20 was produced from synthesis mixtures containing tetramethylammonium hydroxide and fluoride ions from water soluble sources of fluoride such as Na, K and ammonium fluoride.
In U.S. Pat. No. 6,001,328 (Lillerud et.al.), silicoaluminophosphate indicated as UiO-S7 was prepared using tetramethylammonium hydroxide pentahydrate or tetramethylammonium hydroxide in combination with HF.
In a Ph.D. thesis (E. H. Halvorsen, University of Oslo, 1996), it was reported that low silica SAPO-34, designated as UiO-S4, was produced using TEAOH template in combination with HF.
In U.S. Pat. No. 4 503 023 [NH4]2SiF6 is used to post-treat zeolites in order to increase the Si/Al ratio in the zeolite structure. In this case the reagent is used as a source of Si, which substitutes framework aluminum.
Hexafluorosilicic acid, H2SiF6, has been used as a source of Si for the synthesis of zeolite and mesoporous molecular sieves. See, for example, Microporous and Mesoporous Materials, 1999, 27 (2-3), 255-259, Kwon, et al.
As can bee seen from the disclosures described herein, there have been a number of attempts to find alternative synthesis templates for the molecular sieves having the CHA framework type with limited success. It is desirable therefore to find new templates, which are specific for the synthesis of molecular sieves having the CHA framework type. In addition there is a need for new templating systems which afford more effective control of the final composition of the SAPO molecular sieve materials and in particular those that can produce final products with low silica that are relatively pure. A further need is to find methods of preparing low silica SAPO molecular sieves, which do not require the use of hydrogen fluoride, which is toxic, corrosive and volatile.
In the present invention some or all of these requirements have been met by the use of specific fluorine containing compounds that have two or more fluorine substituents, as the source of fluoride ion, in the synthesis of aluminophosphates or silicoaluminophosphates. These sources of fluoride are as effective as hydrogen fluoride but are much easier to handle in the process and provide SAPO""s with the CHA framework type.